/Engine/Source/Editor/LevelEditor/Public/SLevelViewport.h
For the specified actor, toggle Pinned/Unpinned of it's ActorPreview
void ToggleActorPreviewIsPinned ( [TWeakObjectPtr](API\Runtime\Core\UObject\TWeakObjectPtr)< [AActor](API\Runtime\Engine\GameFramework\AActor) > PreviewActor )